A pressure ulcer is created when the skin over a bony prominence is allowed to remain squeezed between that protruding hard anatomy and a firm surface for an extended period of time. The pressure can be as little as 32 mmHg (or the pressure that can be created by firmly squeezing a forefinger and thumb together to create a white print on the squeezed surface of the skin that is visible when the forefinger and thumb are separated again). The time it takes to start an injury may be as little as 20-30 minutes. When circulating blood, which gives the pink color to a individual's fingertips, is prevented from flowing, a clot may form in tiny vessels in the dermis just under the surface of the skin.
The dermis is the second of two layers of the skin. The first layer of the skin above the dermis is called the epidermis. Stopping the blood flow in the dermis deprives the overlying epidermis of the skin of oxygen and nutrients, thereby causing it to die. Repeated prolonged pressure will cause this process to extend to deeper tissues causing the necrosis (death) of the fat, muscle and bone trapped between the protuberant bone and underlying surface.
On the foot, there is little to no cushioning layer of fat, as evidenced by the fact that bones can be easily felt just below the surface of the skin. Because of this lack of a cushioning layer, pressure ulcers can form quickly and extend to the bone with the first episode of prolonged unrelieved pressure. A patient immobilized on a firm surface, even on a hospital mattress designed to lessen surface pressure, will form a clotted capillary network in the dermis at the area of continued pressure over a relatively short period of time.
There are four (4) stages assigned to pressure ulcers, depending on the depth of the necrotic tissue: In Stage I, the capillary bed under the skin surface clots and a red patch develops which cannot be blanched with finger pressure to the center of the reddish blotch. In Stage II, the epidermis overlying the clotted capillary network in the dermis dies and separates causing a blister like wound. In Stage III, the depth of the dead tissue extends through the layers of the skin into the fat beneath. In the foot, there is usually little to no fat under the skin, so these ulcers immediately become Stage III or IV and are classed “unstageable” in feet. Infection of the wound is common in Stage III and Stage IV pressure ulcers, which causes the tissues to turn dark and form a soupy drainage giving off an odor of decaying flesh.